The methodology disclosed herein generally relates to systems and methods for producing composite laminates using tow placement technology. In particular, the methodology disclosed herein relates to the automated design of variable-stiffness composites consisting of plies with spatially varying fiber orientation.
Fiber-reinforced composite materials comprise fibers embedded in a matrix material, such as thermoset and thermoplastic polymer resins. The fibers carry loads and provide strength and stiffness. A tape layer in a composite material has high strength and stiffness in the direction of the fiber, and lower strength and stiffness in a direction perpendicular to the fiber.
Advanced fiber placement (also known as “tow placement technology”) is an automated process for producing composite laminates. Multiple narrow strips (i.e., “tows”) of slit prepreg tape are placed on a surface in bands of parallel tows called “courses”. This technique allows fibers to be curved and tows to be cut and restarted. Where courses converge, overlaps can be eliminated by cutting and restarting tows. Constraints in the manufacturing of fiber-placed composites include a minimum turning radius and a maximum thickness build-up. Typical design practice utilizes plies in which fibers do not change direction, i.e., each fiber is straight within a ply. These designs will be referred to as “conventional” in the rest of this disclosure.
Research on fiber-reinforced composite materials with a varying in-plane fiber orientation has shown that variable stiffness can improve structural performance. The use of steered fibers in composite lay-ups is recognized as a means of tailoring the stiffness and strength in various directions to minimize the weight for the required performance of the structure. Composites consisting of plies with varying fiber orientation are called “variable-stiffness composites”. In most cases, curvilinear fiber paths manufactured by tow placement are used to construct these variable-stiffness composites.
Designs have been produced which include steered-fiber plies. These approaches either have limited design spaces, or may not be manufacturable. For manufacturable designs, one-dimensional fiber path description has been used to optimize buckling load of cylindrical and conical laminates. Hypothetical designs (which may or may not be manufacturable) include: (1) alignment of fibers with principal stress directions done both analytically and experimentally; and (2) variation of fiber angles in two directions in flat panels.
These known methods for designing composite plies involve spatially varying fiber angles. Since fiber-reinforced composite materials usually comprise multiple plies, optimizations for specific loading conditions result in multiple plies with different fiber angle distributions. Steered fiber ply design approaches heretofore have typically represented fiber angle distributions directly (e.g. as Bezier surfaces) which has the drawback that for other than simple angle distributions, it is expensive to compute the thickness build-up associated with a given angle distribution. This leads either to designing potentially unmanufacturable laminates (due to excessive thickness build-up or tow cuts and adds), to the use of the limited design space of a simple angle distribution, which results in less efficient designs than would be produced in a more permissive design space, or to using a computationally expensive inverse solution to produce a stream function for measuring thickness build-up.
There is a need for methods of designing steered-fiber plies that rely on computational optimization techniques to cope with a large design space, while ensuring manufacturability.